A liquid crystal (LC) cell, in general, consists of two substrates forming a cavity between them to contain the nematic LC mixture. Between each substrate and the LC medium, there exist both a conductive electrode coated on the substrate and a LC alignment layer in direct contact with the LC medium to align the adjacent LC directors into one direction. For a TN LC cell, the direction of the LC directors adjacent to one of the substrates is orthogonal (or 90.degree.) to the direction of LC directors adjacent to the other substrate so that the LC directors in the cell twist 90.degree. from one substrate to the other. In the quiescent state, the director of the TN cell twists continuously from 0 to 90.degree. within the LC cell. To the first approximation, the d.DELTA.n value (where d is the cell gap and .DELTA.n is the birefrigence of the LC medium within the cell) of the TN cell is chosen in such a way that if an incident light beam polarized along the entrance LC director, its polarization direction at any given point within the LC cell is parallel to the nematic LC director at that point. In other words, the polarization of the incident light is guided by the LC director within the TN cell so that the output polarization is rotated 90.degree. with respect to the incident polarization. For display applications, the TN cell is placed between two polarizers with the transmitting axes of the polarizers either parallel or perpendicular to the adjacent LC directors. If the transmitting axes of the two polarizers sandwiching the TN cell are crossed to each other, we operate the TN display in a normally-white case where the quiescent state of the TN cell is the bright state of the display. On the other hand, if they are parallel to each other, we operate the TN display in a normally-black case where the quiescent state of the TN cell represents the dark state.
Because of their maturity in manufacture and their sufficiency in performance, TN liquid crystal (LC) displays have been widely used in commercial thin-film-transistor (TFT) driven flat panel liquid crystal displays (LCDs). The strong viewing-angle dependence of the contrast ratio, the brightness, and the grayscale of the TFT-driven TN have been recognized as major weaknesses for these displays. To illustrate the viewing-angle problem of the TN LC cells, we have to define a TN orientation with different viewing directions.
FIG. 1 shows a TN liquid crystal display 10 with two substrates 12 and 14. A rubbed polyimide film (not shown in FIG. 1) is usually used to align the LC directors. The rubbing directions of the polyimide films on substrates 12 and 14 are shown as dashed arrow 16 and solid arrow 18, respectively. For display applications, the TN is placed between two polarizers 20 and 22 with the transmitting axes of the polarizers being either parallel or perpendicular to adjacent LC directors. If the transmitting axes of 24 and 26 of the two polarizers sandwiching the TN LCD are crossed to each other, the LCD is operated in the normally-white mode where the quiescent state of the LCD cells is the bright state of the display. On the other hand, if the axes 24, 26 are parallel to each other, the LCD cells are operated in the normally-black mode. For the normally-white case, there are two optical eigen modes, the ordinary-ray (o-) and the extra-ordinary-ray (e-) modes, in which the optical field propagates either parallel or perpendicular to the nematic LC directors in the TN cell, respectively. Such e- and o-modes are illustrated in FIG. 1 where the transmitting axes of the polarizers 20 and 22 are shown.
With the configuration of the TN display shown in FIG. 1, by facing the display, we can define four viewing zones, the upper viewing zone for viewing from the 12 o'clock direction, the lower viewing zone for viewing from the 6 o'clock direction, the left viewing zone for viewing from the 9 o'clock direction, and the right viewing zone for viewing from the 3 o'clock direction. The sign of the angles for the upper and right viewing zones are positive while those for the lower and left viewing zones are negative. Traditionally, the o-mode has been used for bi-level displays. Recently, Takano et al have carried out a detailed comparison between the o- and e-modes of NW, first-minimum TN cells for analog-gray scale full color displays (H. Takano, M. Ikezaki, and S. Suzuki "Threshold Voltage Biased E-mode TN LCD-Optimum Optical Design for Grayscale Application," the IV International Topical Meeting on Optics of Liquid Crystals, Oct. 7-11, 1991, Cocoa Beach, Fla.). They paid particular attention to optimizing the angular region that preserves a proper grayscale order (no grayscale reversal), i. e., minimizing the angular region of grayscale reversals for ratios of eight gray levels. They concluded that the e-mode with a near threshold-voltage bias is superior to the o-mode for analog-gray scale applications. The following discussion of FIGS. 2 to 5 is in reference to the e-mode. However, the results are applicable to the o-mode as well.
To illustrate the viewing-angle problem of TN for analog-gray scale displays, We show transmittance as a function of applied voltage for a typical TN cell in FIG. 2 when the TN cell is being viewed from five different directions. Curves 1, 2, 3, 4 and 5 in FIG. 2 correspond to viewing from normal incidence, 40.degree. from left viewing zone, 50.degree. from right viewing zone, 30.degree. from lower viewing zone, and 30.degree. from upper viewing zone, respectively, where the angles in degrees are defined as the angles of viewing directions with respect to the normal of the display panel. FIG. 2 illustrates that, at a given voltage applied to the TN cell, the brightness (or the contrast ratio) of the display appears different from above mentioned five different viewing directions.
To further quantify the viewing-angle problems of a typical TN cell, we select eight different voltage levels applied to the TN cell to achieve eight approximately equally-spaced gray levels starting from the brightness to the darkest states of the display. The change of these eight levels as a function of viewing angles in the horizontal and vertical viewing directions are shown in FIGS. 3 and 4, respectively, for a typical TN cell. As shown in FIG. 3, at a horizontal viewing angle of either +40.degree. or -40.degree., the transmittance of the gray level 8 (the darkest level near normal incidence) is higher than that of the gray level 7. Therefore, we have contrast or gray-level reversal between gray level 8 (g8) and gray level 7 (g7) for these viewing directions. The display will appear annoying if a grayscale reversal occurs between any two gray levels from level 1 to level 8. FIG. 5 shows iso-contrast curves as a function of viewing angle for a typical TN display. We can see that the contrast ratio decreases when the viewing angle further deviating from normal incidence. The TN cell usually has the best contrast ratio near normal incidence. FIG. 5 also shows that, outside the thick solid curves, image (or grayscale) reversal occurs so that display appears annoying when it is viewed from those viewing zones having image reversals.
The narrow viewing-angle characteristics of a TN cell are caused by the dark state whose effective retardation changes when the viewing direction is changed away from normal incidence. To the first degree of approximation, the retardation of a nematic LC cell at normal incidence under any applied voltage V is proportional to d.DELTA.n(V) where d is the cell gap and .DELTA.n(V) is the birefringence of the LC medium under an applied voltage V. The transmission of this state between crossed polarizers is proportional to the square of sin(.pi. d.DELTA.n(V)/.lambda.), where .lambda. is the wavelength of the incident light. To accomplish a contrast ratio larger than 200 to 1 at normal incidence, the value of .pi. d.DELTA.n(V)/.lambda. should be less than about 0.07. When the LC cell is viewed from an oblique direction, the retardation becomes either larger or smaller than that at normal incidence so that it may become either brighter or dimmer than that at normal incidence, resulting in viewing the same gray level with different brightness depending on the viewing directions.
Many prior art systems use compensation films attached on TN cells of LCDs to reduce the dependence of effective birefringence as a function of viewing angle, i. e., reducing the variance in relative gray levels over a wide range of viewing angles. However up until now, the compensation mechanisms that have been proposed have been lacking in many respects.